Fine structure of the stretch receptor in the bursa copulatrix of the butterfly, Pieris rapae crucivora.

A pair of multipolar stretch-receptive neurons were found in the bursa copulatrix of the female cabbage white butterfly, Pieris rapae crucivora. The cell body of each neuron, about 10 micrometers in diameter, lies on the edge of the muscular region in the antero-lateral wall of the corpus bursae. No special accessory structure, and as a receptor muscle, is associated with the neuron. The several dendrites extend radially into the muscle layer. The dendrites are ensheathed except for their terminal tips, and, on their course, they anchor repeatedly on the epithelial cells or the muscle fibers in such a manner that their basement membranes fuse together. While the ensheathed dendrite is usually 0.1-0.2 micrometer in diameter, it often forms 1-2 micrometers varicosities especially at anchor sites, so that it looks like a varicose, or beaded, chain. The varicosities contain a number of mitochondria, but only microtubules are found in the fine interconnecting parts of the dendrite. The naked dendritic tips terminate in the basement membrane of the epithelial cell. The varicosities, as well as naked tips, seem to be important for stimulus transduction in the sensory cell of this type.